1. Field of Invention
The present invention relates generally to corrugated feed horn assemblies and more particularly to a corrugated feed horn assembly for use in an antenna assembly having a non-circularly shaped reflector to transmit and receive circularly polarized signals.
2. Description of Related Art
An important design concern for most antennas is their overall size. Smaller antennas are desired for reasons of aesthetics and also for surface mounting requirements. While smaller sized antennas are advantageous, there are associated potential problems with performance caused by their smaller size. Recent advances, however, in the communication satellite industry have made it possible to use smaller antennas in two-way communications, commonly known as VSAT (very small aperture terminal) networks. These antennas typically range in circular aperture size from 60 cm to 4.5 m and provide acceptable performance for most applications.
Another problem in antenna design has been the production of antennas capable of communicating with closely spaced satellites. When satellites have geostationary orbits that are two degrees (2°) or less apart, their respective communication paths are in close proximity to one another when focused by a reflector to the feed assembly of an antenna. Because of this close proximity, there are typically concerns with interference between the two communication links. It is now possible, however, to build a system with antennas having significantly less gain than the conventional 3.8 m reflector antenna satisfying the two degrees (2°) satellite spacing. These antennas allow for communication with closely spaced satellites using one antenna. The solution is either to use larger circular reflectors with higher gain and narrower beam widths, or to use elliptical or rectangular reflector profiles.
The future of the satellite communication industry is leaning toward wider bandwidth to accommodate expanded services at lower cost. The current Ku-band (10.7–14.5 GHz) VSAT communication terminal operates in orthogonal linear polarization configuration to minimize the cross talk and to provide additional isolation between the transmit and the receive ports of an antenna. However, the allocated Ku-band suffers from limited capacity and data transfer speed. The alternative is to utilize the Ka-band (20/30 GHz), which offers wider bandwidth and higher data rate. The broadband technology is instrumental for high-speed interactive IP-based traffic, digital video, and multimedia applications.
On one hand, the satellite spacing requirement demands an elliptic aperture to eliminate cross-talk and to provide higher level of signal isolation at two degrees (2°) adjacency. However, Ka-band satellites are typically designed to operate with circularly polarized signals either Right Handed or Left Handed (RHCP/LHCP) ground terminal. Communication systems that use circularly polarized signals require antennas with circular reflector profiles for total electrical symmetry. Specifically, a circularly polarized signal consists of two vector components that are ninety (90) degrees relative to each other. Further, the vector components have the same magnitude. To maintain the integrity of the signal, the vectors must remain substantially at the same magnitude, and they must remain substantially orthogonal to each other. Circular antenna reflectors maintain this electrical symmetry. Elliptical reflectors, on the other hand, do not because of their lack of symmetry in the horizontal and vertical directions. Consequently, there is a need for reflectors and feed horn assemblies that can accommodate the two degrees (2°) satellite rejection and at the same time operate in a circularly polarized environment.
The combined solution of cross-talk and circularly polarized requirements is an elliptical reflector profile that establishes two way communications links with satellites and functions in a circularly polarized environment. However, as mentioned, the reflector ellipticity destroys the system symmetry and creates a high level of axial ratio, due to reflector aspect ratio. The reflector ellipticity generates phase and amplitude degradation between the two orthogonal electric and magnetic fields. Consequently, it typically results in: (1) generation of extremely high cross-polarization, (2) extensive cross-talks between adjacent satellites, (3) degradation of co-polarized signal, (4) loss of transmit and receive power to the link satellite, (5) lower Effective Isotropic Radiation Power (EIRP), (6) higher system and background noise temperature, and (7) loss of satellite link.